This invention relates to plasma arc cutting systems. More specifically, it relates to a novel electrode, a novel electrode cooling system, and a novel gas cooling process for use with the electrode.
Applicants have devised an apparatus and method for contact starting a plasma arc torch which does not require the torch to touch the metal workpiece being cut or welded by the torch. The torch has an electrode that moves axially within the torch body under the influence of a spring and opposed gas forces which act on the lower surfaces of the electrode adjacent the anode, typically the torch nozzle. On start up, it wa found that gas pressure in the region between the electrode and could build to a sufficient level that they could lift the electrode against the spring. When the gas is cut off to stop cutting, the spring returns the cathode to a position where it contacts the anode and closes off the plasma exit port in the nozzle. This structure and mode of operation are described and claimed in the aforementioned parent U.S. patent application No. 009,508.
This development has been a significant improvement in arc contact starting and has found commercial acceptance. However, several areas for improvement have come to light. One relates to the fact that the electrode must be cooled. In earlier designs, applicants have split the plasma gas stream, directing a portion through a swirl ring to a region over the electrode, as shown in FIG. 1. After circulating around the upper surfaces of the electrode, the gas exits to the atmosphere through ports and passages formed in the torch body. This arrangement has several disadvantages. One is that the presence of pressurized air above the electrodes, as well as below it, produces a delicate balancing of the fluid forces acting on the electrode. This adversely affects the reliability of the separation of the electrode from the nozzle on start up. Another is that this cooling is relatively inefficient since the gas contacts only a fraction of the surface area of the electrode, and at a point most distant from the plasma arc. Finally, this cooling arrangement consumes more plasma gas since the portion used for cooling does not contribute to forming the plasma arc.
Another problem is that the mating surfaces of the housing (which is intended to mean the other housing as well as components which are mounted on the housing and can guide and support the electrode), must be machined to extremely tight tolerances in order to allow a sliding movement of the electrode, while at the same time blocking the flow of pressurized gas past the electrode. The machining requirements to produce parts with the requisite tight tolerances engender higher manufacturing costs, both for the torch as well as for replacement electrodes.
It is therefore a principal object of this invention to provide a cooling system and process for the electrode of a plasma arc torch, particularly one where fluid forces move the electrode for contact starting, which cools more effectively than known systems and does not interfere with the balance of forces acting to create the arc.
Another object is to provide a cooling system and process with the foregoing advantages that requires looser machining tolerances, and therefore a lower cost of manufacture, than comparable known systems and processes.
Yet another object is to provide a cooling system and process with the foregoing advantages that also consumes less plasma gas than comparable known systems and processes.
A still further object is to provide a cooling system and process with the foregoing advantages which can be readily retrofit onto existing plasma arc torches.